FlatGeobuf writing: in SPATIAL_INDEX=NO mode, deal with empty geometries as if there were null

[rouault]

• this fixes the crash when writing an empty polygon. Fixes Segmentation fault when writing an empty polygon to FlatGeobuf #11419
• this avoids a reading error when writing an empty line

@bjornharrtell Can you confirm that FlatGeobuf is not a fan of EMPTY geometries ?

[bjornharrtell]

@rouault confirmed. I either like to pretend they do not exist or treat them as null geometry.

[rouault]

Also add support for writing a file without feature in SPATIAL_INDEX=NO mode ( @bjornharrtell no reason why we should disallow this, right ? since writing empty files in SPATIAL_INDEX=YES mode already works)

[coveralls]

coverage: 74.16% (-0.01%) from 74.172%
when pulling c59ee22 on rouault:flatgeobuf_empty_geoms
into 57833dc on OSGeo:master.

[bjornharrtell]

Also add support for writing a file without feature in SPATIAL_INDEX=NO mode ( @bjornharrtell no reason why we should disallow this, right ? since writing empty files in SPATIAL_INDEX=YES mode already works)

No, must been an oversight that it wasn't doing so already.

[rouault]

The backport to release/3.10 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply e1f2404082... FlatGeobuf writing: in SPATIAL_INDEX=NO mode, deal with empty geometries as if there were null
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging autotest/ogr/ogr_flatgeobuf.py
CONFLICT (content): Merge conflict in autotest/ogr/ogr_flatgeobuf.py
Auto-merging ogr/ogrsf_frmts/flatgeobuf/ogrflatgeobuflayer.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/3.10 release/3.10
# Navigate to the new working tree
cd .worktrees/backport-release/3.10
# Create a new branch
git switch --create backport-11420-to-release/3.10
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick e1f240408258b7505c80582721509110226848dc,c59ee2217869fb3ee5af5be28a48c653ac07f576
# Push it to GitHub
git push --set-upstream origin backport-11420-to-release/3.10
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.10

Then, create a pull request where the base branch is release/3.10 and the compare/head branch is backport-11420-to-release/3.10.

[rouault]

cherry-picked to 3.10 per 340f72f and f3e4b05